The periodic table of elements is an important tool for us to learn chemistry. For example, some information of the periodic table of elements (helium, lithium, beryllium, boron, neon, argon, etc. Omi

The periodic table of elements is an important tool for us to learn chemistry. For example, some information of the periodic table of elements (helium, lithium, beryllium, boron, neon, argon, etc. Omitted) (1) According to the composition of air, nitrogen and oxygen are the main components. It is known that the most abundant element in air is N, and the atomic number of N can be found from the periodic table of elements.

So the answer is: 7;

(2) According to the symbol of elements, it is found that the symbol of elements with atomic number 13 is wrong and should be written as Al;

So the answer is:13; al；

(3) According to the periodic table of elements, the element with atomic number 1 is H, the element with atomic number 8 is O, and the element with atomic number 20 is Ca. The chemical formula of the compound composed of three elements is Ca(OH)2.

So, the answer: ca (oh) 2;

(4) According to the periodic table of elements, it is known that the element with atomic number 16 is S, and the divalent ion formed after the atom gains electrons is S2-.

So the answer is S2.